Many petroleum crude oil and straight run fractions thereof contain one or more compounds of sulfur, nitrogen, heavy metals, halogen material and oxygen whose removal from the petroleum fractions is necessitated for reasons relating to refinery process operations, product quality or environmental considerations. Hydrotreating is one of the methods commonly used in the petroleum refining industry to remove many of these undesirable foreign elements. Sulfur is perhaps the most common of the contaminating elements in crude oil and is found in one form or another in almost all crude oils and straight run fractions. The removal of sulfur has long been necessary for producing products of acceptable quality and in more recent times has become even more important for improving our surrounding atmospere. Desulfurization processes are conventionally conducted by hydrogenation in the presence of a catalyst which promotes the conversion of sulfur impurities to hydrogen sulfide. In catalytic hydrodesulfurization processes, or CHD, high ratios of hydrogen to sulfur containing hydrocarbon charge are preferably maintained since a high hydrogen partial pressure in the reaction zone has a favorable influence on the efficiency of the desulfurization reaction and the life of the catalyst. Accordingly, the CHD reactor effluent stream is typically comprised of a substantial quantity of light gasiform components such as hydrogen, hydrogen sulfide and C.sub.2.sup.- hydrocarbons as well as distillate and gasoline fractions.
Separation of the inorganic sulfur from the rest of the CHD effluent stream is accomplished principally by fractination in a stripper but the composition of the streams has presented problems in the design and operation of an effective stripper. In particular, the presence of hydrogen sulfide and other acid gases must be taken into account in stripper design and operation in order to avoid serious corrosion problems in the stripper and the reboiler which is conventionally used to provide heated stripping vapor. In the presence of hydrogen sulfide the reboiler temperature is limited to a maximum value above which high corrosion may occur. Accordingly, the reboiler temperature limitation dictates a relatively low pressure operation of the stripper in order to create sufficient vapor at the stripper bottom stages to separate the components at the maximum allowable operating temperature. By limiting the stripper operation to relatively low pressures, the quality of the separation is compromised to the extent that valuable light hydrocarbons in the LPG range are carried over with gasiform components such as hydrogen and C.sub.2.sup.- into the CHD offgas stream.
In an alternative type of product stripper, the reboiler is eliminated and a relatively high temperature stripping medium is used, usually steam. This type of unit is, however, undesirable because dehydrating facilities are needed in order to dry the product.
Thus, the reboiler type stripper has the disadvantage that low pressure operation is dictated by the corrosion problems which would arise from the use of higher temperatures at the bottom of the stripper which would otherwise increase the availability of stripping vapors from the CHD effluent stream. On the other hand, the use of high temperature stripping media from external sources usualy requires additional equipment for separation of the stripping medium from the desulfurised product.